The supermassive black hole (SMBH) at the heart of galaxy M87 has been found to spin, causing its jet to oscillate by around 10 degrees. The newly released findings confirm one of the most important features of black holes that astronomers have suspected for a long time but have not previously been able to confirm experimentally.
M87* is the most studied SMBH. It’s not the largest one we know, but at 6.5 billion solar masses it’s far larger than our galaxy’s own and the most massive close enough to study relatively easily. It’s the monster whose portrait became famous in 2019 when telescopes around the world combined to produce an image of its shadow against the accretion disk on which it is slowly feeding.
Even before those images, astronomers had often focused their telescopes on M87*. Data collected by more than 20 radio telescopes worldwide from 2000-2022 reveals an 11-year cycle in the motion of the base of the relativistic jet M87* is spitting out. Individual images lack the detail to prove this, so a team led by Dr Cui Yuzhu of Zhejiang Lab stacked 164 images in two-year bins. This also smoothed out variations caused by instabilities in the jet on periods of less than a year.
A black hole like this, and the jet it produces, is so enormous that no external force we know could cause the jet to swing, so the movements must be a consequence of the black hole itself.
If the black hole is spinning, however, as they are thought to do, movements of the jet’s base on a regular cycle is to be expected, provided the black hole’s spin axis is misaligned with its accretion disk’s rotational axis. This would create precession similar to the one that causes the Earth’s axis to slowly move across the sky, producing a 26,000-year cycle in stars’ apparent locations and accompanying changes in climate.
Spin is thought to be one of the key features of a black hole, along with mass and electric charge. The no-hair theorem holds that these three characteristics tell us everything there is to know about a black hole. This has recently been challenged, but by adding additional factors, rather than denying the importance of spin. The gravitational pull of a black hole is so intense that as they spin they create a moving distortion of spacetime, a process known as frame-dragging. This would occur even with relatively small black holes, for a giant SMBH such as this one frame-dragging effects would be immense.
"We are thrilled by this significant finding," Cui Yuzhu said in a statement. "Since the misalignment between the black hole and the disk is relatively small and the precession period is around 11 years, accumulating high-resolution data tracing M87's structure over two decades and thorough analysis are essential to obtain this achievement."
The authors note that other SMBHs have also been investigated for similar movements, but these have yet to be found. However, they point out that if the movements are smaller, or the period longer, they would be harder to detect, so there is no reason to doubt spin is the norm.
The study is published in the journa Nature